1. Field of the Invention
The present invention relates to a magneto-optic memory medium. More specifically, it relates to a magneto-optic memory medium having an exchange-coupled ferrimagnetic double-layered structure in which a recording film and a reading film are laminated.
2. Description of the Prior Art
A thin film of amorphous rare earth-transition metal alloy (hereinafter abbreviated as RE-TM film) has been used as a memory medium in a magneto-optic disc device since it has suitable characteristics for magneto-optic recording (as disclosed in Japanese Published Unexamined Patent Applications SHO 60-11746/1985 and SHO 57-12428/1982, and U.S. Pat. Nos. 4,390,600, 4414650, and 4489139). RE-TM films disclosed in the above applications are known in that they sufficiently exhibit recording sensitivity and recording density. However, these RE-TM films exhibit a poorer S/N ratio when reading (reproducing) than optical recording materials put into practice as a Read-Only type or Write-Once type.
Reading capability of a magneto-optic memory medium depends greatly upon magneto-optic effects, specifically upon a Kerr rotation angle and Faraday rotation angle. In order to enhance the magneto-optic effects of the RE-TM film, attempts have so far been made to add Bi, Sn, or like elements thereto. In this regard, an exchange-coupled ferrimagnetic double-layered structure for improving the reading capability of the magneto-optic memory medium is known, the structure having a reading film made of a material exhibiting a greater magneto-optic effect and a recording film made of a material suitable for retaining recorded information. See "Magnetization Process of Exchange-Coupled Ferrimagnetic Double-Layered Films", Japanese Journal of Applied Physics Vol. 20, No. 11, November, 1981 pp. 2089-2095.
The recording film in such a double-layered structure requires appropriate Curie temperature for recording and also a high coercive force. On the other hand, the reading film requires the above-mentioned magneto-optic effects. Japanese Published Unexamined Patent Application SHO 63-18544, for example, discloses such an exchange-coupled ferrimagnetic double-layered structure. In this case, TbFeM (M is Cr or Al) is used for the recording film, and GdFeCo for the reading film.
Generally, the greater the magneto-optic effects, i.e., the Kerr rotation angle or Faraday rotation angle increase, the more the reading capability improves. The magneto-optic effects depend generally upon a wavelength of an applied laser beam. However, in the medium of the above-mentioned conventional exchange-coupled ferrimagnetic double-layered structure, the magneto-optic effects decrease with respect to the applied laser beam of a short-wavelength area thereby decreasing the Kerr rotation angle or Faraday rotation angle, resulting in a poor S/N.
Also, there are limitations with regard to improving the recording medium in recording density using a laser beam of a short-wavelength type. Therefore, the wavelength of a laser beam to be used in recording/reproducing is restricted. This causes restrictive materials to be used for the recording medium and unsuitableness of the medium for general purpose.
In short, the conventional magneto-optic memory medium of the exchange-coupled ferrimagnetic double-layered structure has disadvantages in that the reproducing capability is poor with respect to a short-wavelength laser beam and improvement in the recording capacity using a short wavelength laser beam is limited.